This is a Phase I clinical trial of the ATP-sensitive potassium channel opener PNU-83757 in treating erectile dysfunction. The primary objective of the study is to determine the highest tolerable dose of PNU-83757. The secondary objectives are: 1) to determine the systemic pharmacokinetics parameters of PNU-83757; 2) to assess the linearity of the pharmacokinetics; 3) to obtain a preliminary estimate of the effective dose range; 4) and to collect data on the safety of the drug.